The present invention relates to a device for sensing the outer diameter of a roll of web material, such as paper, unrolled from the said roll for the feeding of any suitable operating machine, such as for example a cigarette making machine. More particularly, the diameter sensing device is adapted for the actuation of a control member operating a splicing device which, upon exhaustion of a roll, inserts in the cycle of the operating machine another new roll.
Of course, the control member must be actuated with the highest degree of accuracy, keeping in mind the high operational speeds of the operating machines: in fact, if the control member is actuated with delay, there might arise difficulties in the splicing operation, due to the fact that splicing machines usually require a certain reserve length of web from the roll being exhausted, for effecting a correct splicing operation, with no stresses or ruptures in the web; on the other hand, if the control member is actuated too early, then on the exhausted roll there remains a remarkable quantity of web which could have been used and which instead is discarded.
Roll diameter sensing devices are known in which there is employed a follower arm, capable of swinging towards the axis of the roll, and which is kept tangent to the outer circumference of the roll being unwound: the reduction of diameter of the roll causes the angular displacement of the follower arm which directly or indirectly acts on a microswitch whenever the predetermined minimum diameter has been reached. Other roll diameter sensing devices employ a photoelectric cell and a light emitting source operatively coupled with the said photoelectric cell, the paper roll being interposed between these two elements, so that the light beam will be intercepted by the full roll, while whenever the roll has reached a predetermined minimum diameter, the light beam will act on the cell, thus controlling the splicing operation.
Generally, the above mentioned known roll diameter sensing means are imprecise, since they are sensible to any possible eccentricity of the reel of the roll with respect to the axis of the roll-carrying mandrel. Due to the said eccentricity, the control signal from the control member for the splicing operation can be imparted too early (roll not completely exhausted), or too late (roll already exhausted, with no sufficient reserve length).
According to the present invention, a device for sensing the outer diameter of a roll of web upon unwinding of the said web from the said roll for the actuation of a control member when the diameter of the roll has been reduced to a predetermined minimum diameter, comprises a follower arms capable of swinging movement to-and-fro in the direction of the axis of the roll around a fixed axis and urged in the direction of the said roll axis. The swinging movement of the said follower arm controls the actuation of a control member, and said follower arm carries a feeler element intended to bear against the circumferential surface of the web roll. According to the present invention, the said feeler element is constructed as a gauging fork or snap gauge having a pair of arms which are spaced between each other of a predetermined distance, in such a manner that the clear distance or clearance between the said two arms corresponds to the predetermined minimum diameter of the roll, and that when the two arms swing over the roll after its diameter has, due to unwinding of the web, reached the predetermined minimum diameter, the follower arm acts on the control member.
In this manner, when the effective diameter of the roll has been reduced to the predetermined minimum, independently from any possible eccentricity of the reel of the roll with respect to the roll carrying mandrel, the gauging fork snaps over the residual roll, thus performing an ample swinging motion and safely actuating the control member for the operation of the splicing device or any other device.